The term Black Hat SEO is used widely to describe any and all tactics of search engine optimization which are considered "unethical" or are forbidden or frowned upon by search engines. It's hard to put an exact definition on black hat SEO since its definition is constantly changing. Web optimizers are constantly coming up with new search engine marketing techniques and tricks designed to boost the ranking of their site a few slots, and sometimes these end up falling into the realm of black hat SEO.
Some people object to the term black hat SEO, claiming that anything you can do to get the search engines to boost your rankings is fair game, so long as you don't get caught. However, those against black hat SEO argue that it is responsible for much of the useless information out there on the internet today. Since black hat SEO is driven solely for the purpose of boosting rankings, many people build websites with little or no informative value for users, but are keyword rich or use other SEO techniques and therefore get higher rankings than sites that really do have something to offer the public. Google and the other major search engine companies agree with this argument, and many of them will penalize or ban sites they find using black hat SEO. For this reason, we don't recommend using black hat SEO, especially with the wealth of other effective SEO methods out there.
However, the line between black hat SEO and white hat SEO becomes blurry when you try to nail down a definition. In essence, all SEO is aimed at boosting your rankings through the manipulation of content or other elements, so in a sense, all SEO marketing is black hat, since it is deliberate. But for practical purposes, we can say that anything Google or the other engines outline in their Terms of Service as being against the rules is definitely black hat SEO. Below are some of the most common black hat SEO techniques, which should be avoided.
A link farm is a type of black hat SEO involving forums devoted only to bringing web sites together for the purpose of linking to each other in order to increase their link popularity and Page Rank. This sort of artificial link building is very transparent, and the search engine crawlers will pick up on it immediately and penalize you.
Cloaking is a type of Black hat SEO that serves up different web pages to search engine crawlers than actual human visitors in an effort to boost rankings. Thee pages are often heavily optimized, but are not visible on the actual site and have no value to the site itself. This can also get you heavily penalized.
Similar to Cloaking, Doorway pages act as click-throughs between pages of actual content that serve no real purpose except as SEO tools. They contain extra links and keywords that are useless to the user, but intended to boost rankings.
All of these black hat SEO techniques have been well documented to result in penalties and bans when discovered by search engine crawlers. They are essentially forms of search engine Spam, and with many other effective methods of white hat SEO available, they are really not worth indulging.